Ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene (e.g., polymers derived from vinyl acetate or other vinyl esters of monocarboxylic acids such as poly(ethylene-co-vinyl acetate) (EVA) or copolymers derived from ethylene and an acrylate ester or methacrylate ester such as ethylene n-butyl acrylate (EnBA)), are used in a wide range of applications. For example, EVA is commonly employed in hot-melt adhesives for use in papers and packaging, in conjunction with non-woven materials, in adhesive tapes, in electrical and electronic bonding, in general wood assembly, and in other industrial assembly. EnBA is used in various hot melt adhesive applications, including low application temperature hot melt adhesives. EnBA has a relatively low glass transition temperature Tg as compared to EVA. EnBA based hot melt adhesives can offer higher adhesion even to difficult substrates, better thermal resistance, increased adhesion to metals and glass, and can offer beneficial low temperature use properties.
EVA and ethylene—acrylic ester based thermoreactivable films are a very efficient way to bond various substrates using continuous or discontinuous processes including calandering, press or injection. This technology is widely used to bond fabrics to foams or fabric to fabric in many applications such as automotive or furniture.
In many cases, for example when used in hot-melt adhesive formulations, ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene are processed at elevated temperatures. In these applications, it is important that the polymers exhibit viscosity stability at elevated processing temperatures. For example, in the case of hot-melt adhesive formulations, changes in the viscosity of the adhesive upon incubation at an elevated processing temperature can affect the quantity of adhesive applied to the substrate over time. This inconsistency can jeopardize the quality of an adhesive bond or joint formed using the hot-melt adhesive. In addition, an increase in hot melt adhesive viscosity can be associated with gelling and can contribute to char particle formation in the hot melt adhesive formulation. Char particle formation in the hot melt adhesive formulation can negatively impact hot melt adhesive clean running properties. This can lead to hot melt equipment nozzle obstruction or can aggravate such an obstruction which can increase hot melt equipment downtime. Unfortunately, ethylene copolymers which are copolymers with one or more polar monomers which contain an ester group and which polar monomers can polymerize with ethylene, such as EVA or EnBA, can exhibit limited viscosity stability at elevated temperatures such as in the case of hot-melt adhesive formulations. Without being bound to any theory, ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene can be thermally unstable such as in the case of hot-melt adhesive formulations. At elevated temperatures, they can degrade, leading to crosslinking of the copolymer and an increase in viscosity. By stabilizing the viscosity of ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene (e.g., copolymers derived from ethylene and vinyl acetate or n-butyl acrylate) at elevated temperatures in hot-melt adhesive formulations, the processing of such materials can be greatly improved.